In contrast to monoreactive antibodies which generally are of high affinity and appear after immunization, polyreactive antibodies are naturally occurring antibodies which bind with low affinity to a variety of self and non-self antigens. Over the last couple of years we have concentrated on the cells that produce polyreactive antibodies and found that polyreactive immunoglobulins are expressed on the surface of B cells and serve as antigen receptors capable of binding a number of different antigens. These polyreactive antigen binding B (PAB) cells are identified by their capacity to bind fluorescein-labeled antigens and are quantitated by FACS analysis. Our recent studies have focused on the frequency and property of PAB cells in the early B cell repertoire (i.e., cord blood) and in autoimmune diseases as compared to the normal adult B cell repertoire. We found that the percentage of PAB cells in the cord blood was approximately twice that found in the peripheral blood of adults and that the properties of PAB cells in the peripheral blood of adults as compared to cord blood were similar except for the fact that in cord blood there were about twice as many CD5+ cells as in the peripheral blood of adults. These findings argue that CD5, which was thought to be a marker for polyreactivity, is not directly linked to polyreactivity. Further studies showed that the percentage of PAB cells in patients with Sjogren's syndrome, systemic lupus erythematosus and rheumatoid arthritis was equal to or slightly below that found in the normal adult B cell repertoire. We conclude from these and other studies that PAB cells are a major constituent of the normal B cell repertoire and are the predominant cell type in the newborn B cell repertoire. Antibodies made by B cells are secreted into body fluids where they act extracellularly as a defense against foreign invaders. Antibodies have little if any effect on intracellular pathogens such as viruses. To see if antibodies would be useful in combating intracellular pathogens (e.g., HIV) plasmids containing cDNA encoding single-chain non-neutralizing anti-HIV antibodies to gp41, with or without endoplasmic reticulum or trans-golgi anchor domains, were constructed and used to transfect human CD4+ T cells (Jurkat). These T cells then were infected with HIV. Our experiments showed that the intracellular antibodies bound to and blocked viral maturation. Antibodies with endoplasmic reticulum or trans-golgi anchor domains were effective in inhibiting HIV replication. In contrast, antibodies without anchor domains were secreted into the culture fluid and failed to inhibit HIV replication. To study the effect of antibodies on other viral proteins, HIV p17 was chosen because it is involved in several different steps in the viral replication cycle. Using the strategy described, we expressed cDNA encoding a single-chain antibody to p17 linked to either a cytoplasmic or nuclear anchor domain. Our study showed that the antibody bound specifically to p17 and produced a marked reduction in syncytium formation and viral titer as compared to cells expressing the vector alone. The demonstration that single-chain antibody directed against gp41 or p17 inhibits HIV replication in cultured cells raises the possibility that intracellular antibodies might be therapeutically useful if introduced into human hematopoietic stem cells from HIV-infected individuals. The hope is that when these stem cells mature into HIV-susceptible CD4+ T cells, the expressed intracellular anti-HIV single-chain antibody will protect them from HIV infection. From a broader perspective, the capacity of expressed intracellular antibody molecules to combine with and block the action of specific intracellular proteins provides a powerful tool for elucidating the function of these proteins.